In a gas and steam turbine system, the heat contained in the expanded operating medium or heating gas from the gas turbine is used to generate steam for the steam turbine. The heat transfer takes place in a waste-heat steam generator downstream of the gas turbine, in which usually a number of heating surfaces are arranged for water preheating, steam generation and steam superheating. The heating surfaces are connected to the water-steam circuit of the steam turbine. The water-steam circuit normally contains several, e.g. three, pressure stages, each having an evaporator heating surface.
Several alternative design concepts are possible for the steam generator connected downstream of the heating gas end of the gas turbine as a waste-heat steam generator, i.e. as a once-through steam generator or as a circulating-steam generator. With a once-through steam generator, the heating of the steam generating tubes provided as evaporator tubes causes evaporation of the flow medium in the steam generator tubes in a single passage. In contrast to this, with a natural- or forced-circulation steam generator the circulated water is only partially evaporated during one passage through the evaporator tubes. The water not evaporated is re-supplied to the same evaporator tubes for further evaporation after removal of the generated steam.
A once-through steam generator is, in contrast to a natural- or forced-circulation steam generator, not subject to pressure limitation and therefore live steam pressures far above the critical pressure of water of (PKri≈221 bar) where there are still only slight differences in density between the liquid-similar and steam-similar medium, are possible. A high live steam pressure favors a high thermal efficiency and therefore low CO2 emissions in a power station heated by fossil fuel. In addition, a once-through steam generator is of simpler construction than a circulating-steam generator and therefore can be manufactured at particularly low cost. The use of a steam generator designed on the once-through principle as a waste heat steam generator of a gas and steam turbine system is therefore particularly suitable for achieving a high overall efficiency of the gas and steam turbine system combined with simple construction.
Particular advantages with regard to the cost of manufacture and also the necessary maintenance work is offered by a waste-heat steam generator of horizontal design where the heating medium or heating gas, in particular the waste gas from the gas turbine, is passed through the steam generator in an almost horizontal direction of flow. A horizontal steam generator of this kind is known from EP 0 944 801 B 1. Because of its design as a once-through steam generator, the overflowing of water from the evaporator tubes forming the continuous heating surface into the downstream superheater must be prevented during operation. However, this can cause problems, particularly when starting the steam generator.
When the steam generator is started, a water discharge, as it is called, can occur. This arises when the flow medium in the evaporator tubes initially evaporates due to the heating of the evaporator tubes and this, for example, takes place in the middle of the particular evaporator tube. This causes the quantity of water downstream (also known as water plugs) to be expelled from the particular evaporator tube. To reliably make sure that unevaporated flow medium from the evaporator tubes cannot reach the superheater connected after the tubes, the known steam generator, such as normally also a once-through steam generator of vertical design, is provided with a water-steam separator or precipitator connected between the evaporator tubes forming the continuous heating surface and the superheater. Surplus water is drawn off from this and either returned to the evaporator by a circulating pump or discharged. A water-steam separating system of this kind is, however, comparatively expensive with regard to both design and maintenance.